This invention relates generally to the field of data communications networks and in particular to a method and apparatus for transmitting data at very high rates over multi-mode optical fiber.
Multi-mode optical fiber is perhaps the most widely used type of transmission medium for local area networks (LANs) and as the data rates within LANs are increasing exponentially, multi-mode fiber systems have to support higher and higher data rates. The IEEE 802.3z standard (IEEE 802.3z Standard, xe2x80x9cMedia Access Control Parameters, Physical Layer, Repeater and Management Parameters for 1000 Mb/s Operation,xe2x80x9d June 1998) describes the current state of the art for Gigabit Ethernet at a data rate of 1 Gb/s, or 1.25 Gbaud including the coding overhead specified in the standard.
Even at Gigabit Ethernet rates, the capabilities of multi-mode fiber are stressed. The bandwidth performance of multi-mode fiber is usually specified for xe2x80x9coverfilled launch conditionsxe2x80x9d that were common with light emitting diode (LED) sources. For laser sources which are required at data rates of 1 Gb/s and above, overfill launch conditions are difficult to obtain. In particular, 1000BASE-SX short-wave 850 nm VCSEL (vertical cavity surface emitting laser) Gigabit Ethernet transceivers emit light over an area comparable to the fiber core size and 1000BASE-LX sources at 1.3 micron are single transverse mode and emit only over a small ( less than 10 micron) size area. When these sources are coupled into the center of a conventional multi-mode fiber, performance can vary significantly as described by L.Raddatz et al in an article entitled xe2x80x9cAn Experimental and Theoretical Study of the Offset Launch Technique for the Enhancement of the Bandwidth of Multimode Fiber Links,xe2x80x9d which appeared in the Journal of Lightwave Technology, Vol. 16, pp. 324-331 in 1998.
One of the reasons for this varying performance is the presence of perturbations in the index profile near the core center of many multi-mode fibers which result from the manufacturing process. While overfilled launch excites primarily higher-order modes that do not have much overlap with the center region, a single-mode laser source can couple to the low-order modes in the center of the fiber. Because of the center perturbations, conventional multi-mode fiber can show large xe2x80x9cdifferential mode delayxe2x80x9d (DMD) between low-order modes. Therefore, a laser source that excites primarily low-order modes can cause much lower transmission bandwidth compared to an overfilled launch source in such fibers. At present, center launch is therefore avoided in all multi-mode fiber optic links.
It is worth noting that center-launch has been proposed before as a method to achieve higher bandwidth in multi-mode fibers. (See, e.g, Z. Haas, M. A. Santoro, xe2x80x9cA Mode-Filtering Scheme for Improvement of the Bandwidth Distance Product in Multimode Fiber Systems,xe2x80x9d Journal Lightwave Technology, Vol. 11, pp 1125-1130, 1993) However, the system described therein is vulnerable to bandwidth degradation as long as conventional multi-mode fiber with pronounced center perturbations is used. Also, single-mode accuracy is required to position the laser with respect to the multi-mode fiber, annihilating the alignment cost advantages of multi-mode fiber solutions. Consequently, the same authors therefore concluded that offset launch should be used to increase the bandwidth of the fiber and described that offset launch in U.S. Pat. No. 5,416,862 which issued to Haas, et al. in 1995.
For higher LAN data rates such as 10 Gigabits per second, multi-mode link problems will become even more severe. In particular, even short-wave (850 nm) sources now need to be single transverse mode. This is because multi-transverse mode sources tend to have a spectral bandwidth in excess of 0.2 nm, leading to chromatic dispersion and mode partition noise penalties over the 300-m distances that need to be supported within the Local Area Network.
Modal noise becomes a serious problem for such a high-speed multi-mode fiber link (See, e.g., A. M. Koonen, xe2x80x9cBit Error Rate Degradation in a Multi-mode Fiber Optic Transmission Link Due to Modal Noise,xe2x80x9d Journal of Selected Areas in Communications, SA-4, pp. 1515-1522, 1986; and P. R. Couch, R. E. Epworth, xe2x80x9cReproducible Modal Noise Measurements in Systems Design and Analysis,xe2x80x9d Journal of Lightwave Technology, LT-1, pp 591-595, 1983) As is known, modal noise arises for narrow linewidth sources, due to coherent interference between different mode groups that have different path lengths going through the fiber. This interference is the same phenomenon known as xe2x80x9cspecklexe2x80x9d in optical interferometry. The xe2x80x9cspecklexe2x80x9d will change in time due to mechanical movement of the fiber, or due to mode partition noise of the laser if the laser has more than a single lasing mode. However, as long as the detector captures all light, all variations in the speckle pattern average out.
Modal noise only leads to performance penalties in the presence of mode selective losses, such as occur at dirty, imperfect, or misaligned connectors. Using a 850 nm VCSEL operating with  less than 0.2 nm linewidth, severe modal noise penalties can be observed with the standard overfill launch and 4 connections with a maximum combined loss of 1.5 dB, as specified in the IEEE 802.3z standard for Gigabit Ethernet. These penalties are so large that the link can not operate.
Operation over multi-mode fiber using a laser with narrow spectral bandwidth such as that required for short-wavelength operation (850 nm) at 10 Gb/s, or 1300 nm DFB lasers for a 4-wavelength WDM link operating at 2.5 Gb/s each, therefore is thought to be impossible due to modal noise. Although eliminating multi-mode fiber from consideration for 10 Gb/s operation is a possible solution, multi-mode fiber is preferred for backward compatibility with legacy LAN equipment.
It is therefore highly desirable to overcome the limitations cited above, and provide a stable fiber-optic link solution over multi-mode fiber, capable of operating at data rates on the order of 10 Gb/s for next-generation (10 Gigabit) Ethernet.
We have developed a method and apparatus for sending data at very high data rates (on the order of 10 Gb/s) over multi-mode fiber, for example within premise or local-area networks. According to our method Modal noise penalties in the multi-mode fiber optic link with mode selective losses and spectrally narrow sources such as single transverse mode Vertical Cavity, Surface Emitting Lasers (VCSELs) or DFB lasers are substantially eliminated using restricted center launch into the multi-mode fiber. Even in the presence of offset connectors, mode selective losses then remain small, and modal noise penalties are substantially reduced or minimized.